Tungsten plating and method of producing same



Patented Nov. 1, 1932 COLIN G. FINK AND FRANK L. JONES, OF NEW YORK, N. Y.; SAID JONES ASSIGNOR TO SAID FINK TUNGSTEN PLATING AND METHOD OF PRODUCING SAME N 0 Drawing.

This invention comprises an improvement on that disclosed in application Serial No. 183,237, filed April 12, 1927, by Colin G. Fink for electroplating with metals such as tungsten, thorium, molybdenum, titanium,

&c. which have heretofore been difficult to reduce electrolytically from aqueous solutions of their salts. The present invention consists of certain simplifications and further refinements of the general process described in the aforesaid prior application which have been discovered as the result of a more extensive study of the principles of operation and probable reactions involved therein when applied to tungsten.

In a specific embodiment of the general process described in said prior application a solution of hydroxide of sodium was used with tungstic acid to produce in situ a tungstate, or similar composition of the base metal employed, and to these was added an excess of dextrose, or similar carbohydrate soluble in water to facilitate the electrolysis of the completed bath. Further study and experimentation have developed the fact that the main benefit derived from the use of such carbohydrate was the reduction of the negative exponent of the hydrogen-ion concentration of the solution represented by what is known as the pH number, this modification of the alkalinity of the resultant solution being produced by the gradual decomposition of the dextrose and the consequent liberation therefrom of weak organic acids, thus bringing the degree of alkalinity of the operating .bath to a point which was consistent with the production of the best plating in the shortest possible time when other conditions, such as temperature and current strength, were most favorable. This fact having been established, it then became possible to eliminate the carbohydrate addition, the use of which caused certain minor complications, and to substitute for the hydroxide some other alkali metal salt which, when mixed with a workable proportion of tungst-ic acid, would produce at once a bath of the desired pH number, i. e. about 12.5. The commercial material so far found to best complywith all these requirements is sodium carbonate Application filed May 18, 1929. Serial No. 362,854.

As a general example carrying out the present invention, the following procedure may be outlined: A saturated solution of sodium carbonate (Na CO is made at a temperature of 100 degrees Centigrade and tungstic acid (VVO is added as long as the sodium tungstate thereby produced is soluble. If then the solution is electrolyzed at a current density of (75) seventy-five amperes per square foot of submerged cathode area,

a thin, evenly distributed deposit of substantially pure metallic tungsten will quickly form on the cathode, and this will increase in thickness quite rapidly during a run of twenty minutes or more.

Experiments have shown that the proportions of sodium carbonate and tungstic acid producing the best result are, as at present advised, obtained when a bath is formed in about the proportions of 330 grams of sodium carbonate to 125 grams of tungstic acid per litre of solution. When this is electrolyzed at 100 degrees centrigrade with a platinum anode and brass cathode and a current density of 200 amperes per square foot of submerged cathode area for fifteen minutes, an evenly plated deposit of substantially pure metallic tungsten on the cathode of .128 grams per square decimeter results. The pH number (at the boiling point) of baths ranging from 120 to 200 grams of tungstic acid for every 330 grams of sodium carbonate per litre of solution ran from 13 for the former to 12 for the latter, and good plating resulted from all these mixtures. The best yield of plated metal for current employed, however, is obtained with current densities between 7 5 and 100 amperes per square foot. Satisfactory plating, nevertheless, can be obtained when using current densities such as 400 amperes per square foot, but the higher the current density used, the more quickly a thickmess of deposited layer of metal is obtained at which its begin to form therein. The opera- ;ion the plating bath under all these conditions is accompanied by a constant libera- ;ion of hydrogen bubbles at the surface thereif about the cathode, and calculations have l1OWI1 that less than one per cent of the current consumed is employed in depositmg the ;ungsten, the remainder" being practically all axpended in the evolution of .the hydrogen.

Among the advanta es of this form of the rocess em loying so 'ium carbonate, in ad ition to t ose already mentioned, may be pointed out the fact that all the materials can 3e easily obtained commercially in sufliciently pure states, that the bath is easily made up and has no corrosive action if spilled on the )perators skin or garment, and that it evolves 1o hurtful or offensive vapors in operation. The bath slightly deteriorates with long con- ;inued use. Plated cathodes must be quickly removed from the bath and washed, as otherwise the deposit plated thereon will be dissolved by chemical action of the hot bath.

Potassium hydroxide (KOH) and potas- ;ium carbonate (K 00 solutions may be substituted for the corresponding sodium :ompounds mentioned in the description of she foregoing process, or in the process of the Fink applicatlon above noted,.and equally good results will then be obtained.

When a suspension of lithium carbonate in water is boiled with tungstic acid a solution of lithium tungstate is formed. This solu- 'aion when electrolyzed at 100 degrees centigrade with a cathode current density of 100 amperes per square foot will also produce a plated deposit of metallic tungsten on the :athode. When electrolysis is first started aome parts of the cathode become covered with a white, insoluble coating. No metallic tungsten is deposited on the areas so covered, but the remainder of the cathode receives a oright coating of metallic tungsten. After the bath has been used a few times, these insoluble films no longer form and tungsten plating then becomes uniform and is quickly produced. The action of this bath is then similar in other respects to that of the sodi- 1m-carbonate-tungstate solution.

Ammonium hydroxide is too unstable a ;ubstance at temperatures above degrees :entigrade to be used commercially as the basis of a plating solution, but as it also reacts with tungstic acid to form basic tungstates, such a solution can be used in carrying )ut the process of this invention. A solution )f ammonium hydroxide is heated to 80 degrees centigrade and tungstic acid added bill a slight cloudiness shows that the solu- ;ion is saturated. The solution formed under such conditions contains ammonium oara-tungstate, 5(NI-L) O.12\VO When his solution is electrolyzed at a current density of 100 amperes per square foot using a platinum anode and a brass cathode, a. satis* factory plating of metallic tungsten is produced.

Tungsten plating has also been effected by the above described process on cathodes of copper, zinc, iron, and carbon. Brass, however, forms the most convenient cathode base because of ease in cleaning and polishing its surface before use. I

' The average plated deposit produced by the above described process using sodium carbonate will withstand immersion in concentrated hydrochloric acid for six hours, but

will strip from the cathode base if so im-. mersed for a longer period. It is resistant to the action of alkaline solutions at room temperature, but is slowly dissolved in hot alkaline solutions. However, as a plating of any other common metal would have a life of only a few seconds under any of the above recited conditions, the robust character and practical value of the tungsten plating produced by this process are evident. The deposit is bright, having a high reflecting power and a decorative value equal to chromium.

said cathode surface had before being sub-v jected to the plating operation. This sharply differentiates the product of this invention from the tungsten platings heretofore produced by fused bath processes, which are believed to be the only methods by which adherent platings of substantially pure metallic tungsten have ever heretofore been produced.

WVhile it is understood that the novelty of the herein described process is patentable whether or not the correct theory of its operation is explained, or even known in'detail, the principles involved, so far as at present understood, are as follows: 7

So many theories have been proposed to show why tungsten could not be plated from aqueous solutions that most investigators have neglected the common tungsten compounds to try the more uncommon compounds for plating. It is only within the last few years that the traditional ideas of ionization and valence have been modified to the point Where the very complex compounds have been recognized as special cases. The work of WVerner and others has shown that the compounds of tungsten are not simple aggregates of atoms all joined by the same type of valence bonds but that the more complex of these compounds are really compounds of groups of atoms, these groups being joined by residual valences which do not have the properties of the ordinary valence bond. The complex compounds of this class ionize into complex ions which often differ greatly from those which would be expected from an inspection of the formula as ordinarily written. Dr. Robert Schwarz in his Chemistry of the Inorganic Complex Compounds says in this connection, It must be remembered that in electrolytic analysis as well as in electroplating soluble complex compounds are often used because the precipitates produced from their solutions are particularly even and tenacious. For example, gold is precipitated from the chloride only as a brown powder, while from a solution of the complex salt (Au(CN) )K it separates as a tenacious and lustrous coating. This is also the case with silver as well, which will deposit from acid solutions in which it occurs as a positive ion in the form of loose needle crystals but which plates from a solution of Ag(CN) .K(CN) or as it is best Written (Ag(CN) K, as ,a smooth coherent coating. The assumption of Koerner and other investigators that tungsten would go entirely to the anode when alkaline tungstates were electrolyzed is contrary to both fact and theory.

There are several facts that should be considered in attempting to formulate a theoretical explanation of tungsten deposition. The fact that coherent metal is formed if the solution is at the correct alkalinity and is not formed if the solution is more acid or more alkaline is probably due to the existence at that pH value of the particular complex ion which will deposit tungsten in the form of metal. Other solutions have been made up having the same pH as the sodium tungstate and containing a tungsten salt which would not give a deposit. Plating baths which have been used for some time lose. their ability to deposit tungsten in the form of metal even if the pH is kept constant. These exhausted baths may be allowed to evaporate to dryness and if they are then redissolved in water they are again useful as plating baths. This would indicate that it is the compound formed rather than the alkalinity at which it forms that is necessary for a good deposit. The improvement in plating quality of a solution kept near the boiling point over that of the same solution at a lower temperature and the impossibility of getting a bright coherent deposit below about fifty degrees can be attributed to the fact that the reduction of tungsten to metal is favored by high temperatures. It is impossible to form tungsten by this process without the simultaneous production of hydrogen. The hydrogen may have a chemical function in aiding the reduction of the tungsten compound to metal and if this is the case the use of higher temperatures would favor reduction. The temperatures employed in &

reducing tungsten oxide with dry hydrogen gas are not so much higher than those used in plating but that the action of the hydrogen, especially in the nascent condition, can be considered as the means of reducing the complex tungsten ion which is present. Another explanation which is perhaps more in accord with the results obtained in depositing other metals is that the three metals (Na, Wand H) in the solution are plated out simultaneously in the proportions which would be expected from a study of the theoretical deposition voltages of each. It is quite possible to plate an alloy of copper and zinc from a solution containing both ions. The metal with the lowest deposition potential will have a tendency to form in greatest amounts in such a mixed solution but by selecting the proper concentrations of the different ions and the right current density any desired proportion of the various metals can be obtained. In the solutions used in the herein described process there are present in the order of their potentials hydrogen,-tungsten, and sodium. The hydrogen has the greatest tendency to plate from the solution but in the solutions used the hydrogen ion concentration is very low. The amount of tungsten formed is about one half of one per cent. of the amount which would be expected if all the current went into its deposition and the remainder is used in plating out hydrogen. This would explain the pitting action when a certain amount of plating has been carried out for the formation of small amounts of, metallic sodium, even in very minute amounts, and its immediate reaction with the water of the solution would cause an uneven surface to develop. A study of the voltage across the plating bath and the voltage at the cathode shows a rise in the over voltage with increase of temperature of the bath. This would favor the deposition of the tungsten and account for the better results at higher temperatures. It is unfortunate but never the less true that no study of a plating solution proper can show the conditions and reactions to be found at the surface of the cathode. The hydrogen ion concentration at the cathode surface is probably very much lower than that of the remainder of the solution and the sodium ion concentration much higher. Without the knowledge of these factors it is impossible to calculate whether or not the rate of deposition of tungsten follows the same laws as the deposition of mixtures of metals such as copper and zinc.

While the foregoing disclosure and discussion in detail have dealt with processes operating on tungsten, it should be stated that the general process and procedure herein set out apply with equal success to similar metals, heretofore found difficult to reduce electrolytically from aqueous solutions, the detail formulae for each varying slightly, as may be found to produce the most rapid and eflicient operation.

As an example of our process as successfully applied in the thorium plating, we may prepare a bath containing, er litre, 53 grams of sodium carbonate M 80 21 grams of thorium oxide (ThO in suspension, 60 grams of dextrose (G l-I 0 and subject it to a current density of about 36 amperes per square foot of submerged cathode surface, at from 3 to 5 volts, said bath being maintained at a temperature of about 80 centi 'rade. If a trace of nickel hydrate (Ni()H) is added the thickness of plating obtainable is increased.

In carrying out the process in aluminum plating we prefer to use 150 grams sodium aluminate (Na.A1 0.), 75 grams of dextrose and 40 grams'of sodium carbonate (Na-03) per litre of bath solution and electrolyte at a temperature of about 80 C. with current densities the same as in the case of the tungsten plating.

We have also obtained a satisfactory plating of titanium by using a bath containing, per litre, 10 grams of titanium oxide (TiO in suspension, 15 cubic centimeters of hydrofluoric acid (HF) chemically pure, or 30 cubic centimeters of the commercial 50% solution of such acid, 10 cubic centimeters of hydrochloric acid (HCl), and 0.5 gram of gelatine. This titanium bath we submit to a current density of about 1 ampere per square inch of submerged cathode area, at a voltage of from 4 to 6 volts, maintaining the bath at a temperature of about 80 centigrade.

Good titanium plating can also be produced at lower bath temperatures if the voltage is somewhat increased. As before stated the joint work from which this present invention has been developed has emphasized the fact that the higher the bath temperatures, the more certain and rapid is the desired electrodeposition of the metal, but plating can generally be obtained at bath temperatures below 60 C. by the exercise of care and patience and the proper balancing of other factors with that of heat, inclusive of the control and regulation of the hydrogen film on the cathode surface.

We have obtained bright, adherent deposits of zirconium from aqueous baths operated at 80 C. containing 10 per cent sodium carbonate in which we suspended an excess of zirconium hydrate (Zr(OH) and to which bath we added sulfuric acid (H 80 in small quantities till the hydrogen ion concentration was brought to a point corresponding to a pH of 5.0 to 5.2.

Having described our invention, we claim:

1. An electrolytic plating bath comprising an aqueous solution of tungstic acid and an alkali metal carbonate such as sodium carbonate, the hydrogen-ion concentration of which solution is represented by a pH number of about 12.5.

2. An electro plating bath comprisin an aqueous solution of atungstate formed rom a mixture of tungstic acid and an alkali metal carbonate, the hydrogen-ion concentration of which solution is represented by a pH number between 10 and 13.

3. An electro plating bath comprising an aqueous solution of a tungstate formed from a mixture of tungstic acid andan alkali metal carbonate which solution has a temperature near the boiling point and a hydrogen-ion concentration represented by a pH number between 10 and 13.

4. An electro plating bath comprising an aqueous solution of a tungstate formed from a mixture of tungstic acid and an alkali metal carbonate in the proportions of about 125 parts by weight of tungstic acid to 330 parts by weight of the carbonate per litre of volume of the solution.

5. The process of electro plating with tungsten which comprises forming an aqueous bath of predetermined alkalinity by adding tungstic acid and an alkali metal carbonate to water and subjecting said bath to electrolysis while maintained at a temperature between 60 and 100 degrees centigrade.

6.- The process of electroplating with tungsten which comprises forming an aqueous bath of predetermined alkalinity by adding tungstic acid and an alkali metal carbonate to water and subjecting said bath to electrolysis by a current of about 200 amperes per square foot of submerged cathode surface while maintained at a temperature between 60 and 100 degrees centigrade.

7. The process of electroplating with tungsten which comprises forming an aqueous bath by adding tungstic acid and sodium car-' bonate thereto in proportions which produce a hydrogen-ion concentration represented by a pH number of about 12.5, heating said bath to a temperature between 60 and 100 degrees centigrade, and subjecting it to electrolysis by a current sufiicient to produce a fairly copious evolution of gas bubbles at the surface about the cathode.

8. A process of electro plating with tungsten which comprises adding to an aqueous bath about 125 grams of tungstic acid and about 330 grams of sodium carbonate per litre of bath volume, and subjecting said bath to electrolysis by a current sufficient to produce a fairly copious evolution of gas bubbles at the surface about its cathode.

9. A method of coating with tungsten which comprises adding finely divided tungstic acid and a suitable alkaline reagent to an aqueous bath maintained at a temperature above about 60 degrees centigrade and below boiling point, and inserting the material to be coated in the solution so formed as a cathode and depositing tungsten thereon by electricity.

10. A process of electroplating with metals such as tungsten, thorium, titanium, zirconium and aluminum heretofore found difiicult to reduce in aqueous baths, which comprises forming an aqueous bath of predetermined alkalinity by adding to water an oxide of the metal selected and an alkaline reagent, heating said bath to a temperature above about degrees centigrade and below boiling point and passing through it an electric current, under conditions such that less than one per cent. of said current is expended in glating out the metal, the rest of the current eing consumed in the evolution of hydrogen from the bath.

- COLIN G. FINK.

FRANK L. JONES. 

